Compressor

ABSTRACT

A compressor has a valve port plate made of a steel, through which heat of compressed gas having a relatively higher temperature is transmitted to suction gas having a relatively lower temperature. The valve port plate is nitrided or nitrocarburized for reducing heat transmission from the compressed gas to the suction gas.

BACKGROUND OF THE INVENTION

The present invention relates to a compressor having a valve port platemade of a steel.

In a piston type compressor, a piston is received in a cylinder borewhich is formed in a cylinder block and a housing is connected to theend surface of the cylinder block through a valve port plate and hasformed therein a suction chamber and a discharge chamber. The valve portplate has formed therethrough a suction port and a discharge port. Therotation of a rotary shaft of the compressor is converted through adrive mechanism into the reciprocation of the piston. The reciprocationof the piston causes refrigerant gas in the suction chamber to beintroduced through the suction port into a compression chamber in thecylinder bore for compression therein, and the compressed refrigerantgas is discharged through the discharge port into the discharge chamber.

The discharge chamber in the housing is heated to a high temperature bythe compressed refrigerant gas (discharged gas). Therefore, alow-temperature refrigerant gas flowing from external refrigerantcircuit into the suction chamber is heated by heat transmitted throughthe wall surfaces of the housing and the valve port plate whichcooperate to define the discharge chamber and the suction chamber. Therefrigerant gas in the suction chamber is heated to expand before it isintroduced into the compression chamber of the cylinder bore. Thisresults in a decrease in the amount of refrigerant gas thatsubstantially flows into the compression chamber and hence causes adecrease in volumetric efficiency of the compressor. If the suctionrefrigerant gas is thus heated, the temperature of the gas compressed inthe compression chamber also increases, accordingly. Thus, there hasbeen a problem that a seal member for the compressor or the refrigerantcircuit tends to be degraded by the heat.

A solution for the above problem is disclosed by Unexamined JapanesePatent Publication No. 5-164042, according to which thermal insulationmeans is provided in a partition wall between suction chamber anddischarge chamber of a piston type compressor. As shown in FIG. 8 of theabove-cited Publication, the compressor has a housing 53 having formedtherein a suction chamber 51 and a discharge chamber 52 and connectedthrough a valve port plate 57 to the end surface of a cylinder block 54of the compressor. The valve plate assembly 57 has formed therethrough asuction port 55 and a discharge port 56. The suction chamber 51 and thedischarge chamber 52 are partitioned by a partition wall 58 which hasformed therein a thermal insulation groove 58 a as a thermal insulationmeans.

A compressor disclosed in Unexamined Japanese Utility Model PublicationNo. 2-31382 is provided with a cylinder head which has formed therein asuction chamber and a discharge chamber on one end of a cylinder and ismade of a material having a higher heat radiation, and the suctionchamber in the cylinder head is formed by a thermal insulation material.

A rotary fluid compressor disclosed in Unexamined Japanese PatentPublication No. 5-33119 is provided with a vane-shaped steel materialwhich has formed on the surface thereof an ion-nitriding layer forenhancing abrasion resistance of a vane used for the compressor.

The compressor in the above-cited Publication No. 5-164042 isdisadvantageous in that the housing 53 is different in structure from ahousing of conventional compressor because the thermal insulation groove58 a is formed in the partition wall 68, with the result that anexisting housing is not usable in a compressor. Furthermore, thecompressor disclosed in the above-cited Publication No. 2-31382, whosesuction chamber is formed by a thermal insulation material, requires thestructure of suction chamber to be changed accordingly if a conventionalhousing is to be used for the compressor.

Also, the compressor (vane compressor) disclosed in the above-citedPublication No. 5-33119, whose vane surface is ion-nitrided, is directedto enhance abrasion resistance of steel material, which is aconventional usage of the ion nitriding. Additionally, the above-citedPublication No. 5-33119 does not disclose or teach anything aboutnitriding for decreasing a thermal conductivity of steel material.

The present invention is directed to provide a compressor which preventsan increase in temperature of suction refrigerant gas while improvingthe compression efficiency thereof by providing an appropriate treatmentto the low-cost ferrous material valve port plate without structuralchange to any part of the compressor.

SUMMARY OF THE INVENTION

In accordance with the present invention, a compressor has a valve portplate made of a steel. The valve port plate is nitrided ornitrocarburized.

Other aspects and advantages of the invention will become apparent fromthe following description, taken in conjunction with the accompanyingdrawings, illustrating by way of example the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel areset forth with particularity in the appended claims. The inventiontogether with objects and advantages thereof, may best be understood byreference to the following description of the presently preferredembodiments together with the accompanying drawings in which:

FIG. 1 is a longitudinal cross-sectional view of a variable displacementpiston type compressor according to a preferred embodiment of thepresent invention;

FIG. 2 is a partially enlarged longitudinal cross-sectional view arounda valve plate assembly of FIG. 1;

FIG. 3 is a cross-sectional view that is taken along the line I-I inFIG. 1;

FIG. 4 is a partially enlarged schematic cross-sectional view of anitrided valve port plate according to the preferred embodiment of thepresent invention;

FIG. 5 is a graph showing the relation between a thickness of nitridelayer and a thermal conductivity according to the preferred embodimentof the present invention;

FIG. 6 is a partially enlarged longitudinal cross-sectional view arounda valve plate assembly of a compressor according to an alternativeembodiment;

FIG. 7 is a partially enlarged longitudinal cross-sectional view arounda valve plate assembly of a compressor according to an alternativeembodiment;

FIG. 8 is a partially longitudinal cross-sectional view of a compressoraccording to an alternative embodiment; and

FIG. 9 is a partially longitudinal cross-sectional view of a compressoraccording to a prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe a preferred embodiment of a variabledisplacement piston type compressor 10 according to the presentinvention which is used for a refrigerant circuit in a vehicle airconditioner with reference to FIGS. 1 through 5.

Referring to FIG. 1 showing a longitudinal cross-sectional view of thevariable displacement compressor 10, in which the left side and theright side of the drawing correspond to the front side and the rear sideof the compressor 10, respectively, the compressor 10 has a housing,which includes a cylinder block 11, a front housing 12 and a rearhousing 14. The front housing 12 is fixedly connected to the front endof the cylinder block 11. The rear housing 14 is fixedly connectedthrough a valve plate assembly 13 to the rear end of the cylinder block11.

The housing has formed therein a crank chamber 15 between the cylinderblock 11 and the front housing 12. Between the cylinder block 11 and thefront housing 12 is rotatably supported a drive shaft 16 for extensionthrough the crank chamber 15. The drive shaft 16 is operativelyconnected to a vehicle engine (not shown) for rotation thereby in arrowdirection R.

In the crank chamber 15, a substantially disc-shaped lug plate 17 issecured to the drive shaft 16 for integral rotation therewith. In thecrank chamber 15, a swash plate or a cam plate 18 is accommodated. Theswash plate 18 has formed at the center therethrough a through hole 18a, through which the drive shaft 16 is inserted. Between the lug plate17 and the swash plate 18 is interposed a hinge mechanism 19. The swashplate 18 is connected to the lug plate 17 through the hinge mechanism 19and supported by the drive shaft 16 through the through hole 18 a. Thispermits the swash plate 18 to rotate integrally with the lug plate 17and the drive shaft 16 and incline with respect to the drive shaft 16while sliding in the direction of the axis T of the drive shaft 16.

The cylinder block 11 has formed therein a plurality of cylinder bores20 (only one of them being shown in FIG. 1) around the drive shaft 16 atequiangular spaced intervals, extending in the direction of the axis T.Each cylinder bore 20 receives therein a single-headed piston 21 forreciprocation. The front and rear openings of the cylinder bore 20 areclosed by the piston 21 and the valve plate assembly 13, respectively.In each cylinder bore 20, a compression chamber 22 is defined, thevolume of which varies in accordance with the reciprocation of thepiston 21.

The piston 21 engages with the outer periphery of the swash plate 18through a pair of shoes 23. The housing has formed therein a suctionchamber 24 and a discharge chamber 25 between the valve plate assembly13 and the rear housing 14.

The valve plate assembly 13 includes a valve port plate 26, a suctionvalve plate 27 provided on one side of the valve port plate 26 adjacentto the cylinder block 11, and a discharge valve plate 28 provided on theother side of the valve port plate 26 adjacent to the rear housing 14.As shown in FIGS. 1 and 3, the valve port plate 26 has formedtherethrough suction ports 29 and discharge ports 30. The suction ports29 are located in radially outward positions of the valve port plate 26in correspondence with the respective cylinder bores 20. The dischargeports 30 are located radially inward of the suction port 29 incorrespondence with the respective cylinder bores 20. The suction valveplate 27 has formed therein suction valves 31 in correspondence with therespective suction ports 29. The discharge valve plate 28 has formedtherein discharge valves 32 in correspondence with the respectivedischarge ports 30. The degree of opening of the discharge valves 32 isregulated by a retainer 33 which is fixed to the valve port plate 26.

The compressor 10 has a bleed passage 34, a supply passage 35 and acontrol valve 36 in the housing. The bleed passage 34 connects the crankchamber to the suction chamber 24. The supply passage 35 connects thedischarge chamber 25 to the crank chamber 15. The control valve 36 whichis a known electromagnetic valve is located in the supply passage 35.

The valve port plate 26 will now be described more in detail. The valveport plate 26 is made of steel (electromagnetic soft steel in thepreferred embodiment) and nitrocarburized to have a thermal conductivityof 60 W/mK or less. The valve port plate 26 has nitride layers 26 aformed on both front and rear surfaces thereof, as shown in FIG. 2. Whenthe valve port plate 26 is nitrided, a base material 37 of the valveport plate 26 has formed on the surface thereof a nitride layer 26 a andalso has formed at a deeper portion than the nitride layer 26 a adiffusion layer 37 a, as shown in FIG. 4. The diffusion layer 37 a isnot illustrated in FIGS. 1 and 2 for the sake of convenience ofillustration.

A thermal conductivity of the valve port plate 26 which has formedtherein the nitride layers 26 a depends on a method for forming thenitride layer 26 a and a thickness of the nitride layer 26 a, which willbe described later. For example, the nitride layers 26 a are formed bysalt-bath nitriding to have a thickness of 20 μm or more. The valve portplate 26 is, for example, formed to have a thickness of 2 to 3 mm. Anincrease in temperature of suction refrigerant gas can be prevented moreeffectively by increasing the nitride layer thickness and, therefore,forming the thicker nitride layers 26 a is preferable in terms of theprevention of an increase in temperature of suction refrigerant gas.However, the thicker nitride layers 26 a require a longer time fornitrocarburizing and hence more treatment cost than the thinner one. Thethickness of the nitride layers 26 a in the preferred embodiment hasbeen determined by the trade-off between the effectiveness to preventtemperature rise of the nitride layers and the treatment cost thereof.

The following will describe the relation between a thickness of thenitride layer formed by salt-bath nitriding and a thermal conductivityof a material for nitriding. The salt-bath nitriding was performed by aknown method. The salt-bath mainly contains cyanate. Using sodiumcyanate (or NaCNO) or potassium cyanate (or KCNO) as cyanate, a materialfor nitriding was nitrided at a temperature of 580 to 600 degrees C. Thenitriding was performed using unpolished electromagnetic soft iron platehaving a thickness of 1 mm as the material for nitriding. The resultsare shown in FIG. 5.

FIG. 5 shows the results, as measured when gas nitrocarburizing wasperformed as nitriding. Electromagnetic soft iron was used as thematerial for nitriding. The gas nitrocarburizing was performed at atemperature of about 580 degrees C. The results are also shown in FIG.5. In FIG. 5, triangular dots (or “Δ”) indicate the results in salt-bathnitriding, and quadrangular dots (or “□”) indicate the results in gasnitrocarburizing.

FIG. 5 confirms that an increase in thickness of the nitride layer(compound layer) results in a decrease in thermal conductivity of avalve port plate as a whole. FIG. 5 also confirms that a thermalconductivity depends on which nitriding is performed for forming anitride layer having substantially the same thickness. If a nitridelayer is formed with salt-bath nitriding, the results show a largerpercentage of decrease in thermal conductivity relative to an increasein thickness of the nitride layer in comparison to a nitride layerformed by gas nitrocarburizing. A required thickness of the nitridelayer should be 20 μm or more in salt-bath nitriding to gain a thermalconductivity of 60 W/mK or less. In contrast, the nitride layer formedby gas nitrocarburizing requires twice as thick as the nitride layerformed by salt-bath nitriding.

A coefficient of thermal expansion of a nitrided product was measured,and it showed a substantially equivalent value to a non-nitridedproduct. This is because the thickness of the nitride layers is thinenough.

Surface hardness of the nitride layers was measured. The nitride layerhaving a thickness of 19 μm has a surface hardness of about 675 insalt-bath nitriding, while the nitride layer having a thickness of 20 μmhas a surface hardness of about 580 in gas nitrocarburizing.

The following will describe the operation of the compressor.

As the drive shaft 16 is rotated, the swash plate 18 rotates therewith,and the rotation of the swash plate 18 is converted through a pair ofthe shoes 23 into the reciprocation of each piston 21 in its associatedcylinder bore for a stroke length corresponding to the inclination angleof the swash plate 18 (which the swash plate 18 makes with a planeperpendicular to the axis T of the drive shaft 16). Thus, refrigerantgas is drawn from the suction chamber 24 into the compression chamber 22for compression therein, and the compressed refrigerant gas isdischarged into the discharge chamber 25, repeatedly. As the piston 21moves from the top dead center toward the bottom dead center, therefrigerant gas in the suction chamber 24 (carbon dioxide in thepreferred embodiment) flows into the compression chamber 22 through thesuction port 29 while pushing open the suction valve 31. As the piston21 moves from the bottom dead center toward the top dead center, on theother hand, the refrigerant gas introduced into the compression chamber22 is compressed to a predetermined pressure and discharged into thedischarge chamber 25 through the discharge port 30 while pushing openthe discharge valve 32. The refrigerant gas discharged into thedischarge chamber 25 is sent to the external refrigerant circuit througha discharge hole (not shown).

The control valve 36 is operable to control the opening degree thereoffor adjustment of the balance between the amount of high-pressuredischarged gas through the supply passage 35 into the crank chamber 15and the amount of gas from the crank chamber 15 through the bleedpassage 34, thus determining the pressure in the crank chamber 15. Asthe pressure in the crank chamber 15 is varied, the pressure differencebetween the crank chamber 15 and the compression chambers 22 across thepistons 21 is varied, accordingly, so that the inclination angle of theswash plate 18 is altered, thereby changing the stroke of the piston 21and hence the displacement of the compressor 10.

For example, a decrease in the pressure in the crank chamber 15increases the inclination angle of the swash plate 18, therebyincreasing the stroke of the piston 21, resulting in an increase in thedisplacement of the compressor 10. On the other hand, an increase in thepressure in the crank chamber 15 reduces the inclination angle of theswash plate 18, thereby reducing the stroke of the piston 21, resultingin a reduction in displacement of the compressor 10.

In operation of the compressor 10, compressed refrigerant gas istemporarily reserved in the discharge chamber 25 under high pressure andtemperature. If the valve port plate 26 is made of non-nitrided ornon-nitrocarburized cold-rolled steel plate or made of non-nitrided ornon-nitrocarburized electromagnetic soft iron having a thermalconductivity of about 80 W/mK, the heat of the refrigerant gas in thedischarge chamber 25 is easily transmitted through the valve port plate26. Accordingly, the refrigerant gas in the suction chamber 24 orpassing through the suction port 29 is heated, resulting in a decreasein the amount of refrigerant gas substantially introduced into thecompression chamber 22, thus reducing a volumetric efficiency of thecompressor.

However, the valve port plate 26 of the preferred embodiment is sonitrided to have formed thereon the nitride layers 26 a that the valveport plate 26 has a thermal conductivity of 60 W/mK or less as a whole,with the result that transmission of the heat of the refrigerant gas inthe discharge chamber 25 through the valve port plate 26 to therefrigerant gas in the suction chamber 24 is prevented. Additionally,the suction refrigerant gas passing through the suction port 29 isprevented from being heated, so that the amount of the refrigerant gassubstantially introduced into the compression chamber 22 is increasedand the volumetric efficiency and compression efficiency of thecompressor are improved, accordingly.

When the valve port plate 26 is nitrided (nitrocarburized), the valveport plate 26, that is, the base material 37, has formed on the surfacethereof the nitride layer 26 a, while having formed therein thediffusion layer 37 a of nitrogen contiguously to the nitride layer 26 a,as shown in FIG. 4. The nitride layer 26 a and the diffusion layer 37 acooperate to contribute to a decrease in thermal conductivity of thevalve port plate 26 and an increase in surface hardness thereof.

In the preferred embodiment, electromagnetic soft iron is used as amaterial for nitriding in both salt-bath nitriding and gasnitrocarburizing but low-carbon steel plate such as SPCC (or steel platecold commercial), SPCD (or steel plate cold deep drawn), and SPCE (orsteel plate cold deep drawn extra) may also be used.

According to the preferred embodiment, the following advantageouseffects are achieved.

-   (1) The valve port plate 26 of the compressor 10 has formed thereon    the nitride layers 26 a to have a thermal conductivity of 60 W/mK or    less. Accordingly, the valve port plate 26 made of an inexpensive    ferrous material may also have a thermal conductivity of 60 W/mK or    less, so that transmission of the heat of the refrigerant gas in the    discharge chamber 25 through the valve port plate 26 to the    refrigerant gas in the suction chamber 24 is inhibited, thus    preventing an increase in temperature of the suction refrigerant gas    and improving the compression efficiency of the compressor. The    valve port plate 26 by the nitriding (nitrocarburizing) is made    harder and, therefore, may be made thinner in comparison with a    non-nitrided valve port plate.-   (2) Since the valve port plate 26 is salt-bath nitrided, a thickness    of nitride layer 26 a required for a target thermal conductivity or    less of the valve port plate 26 can be formed more quickly in    comparison to gas nitrocarburizing. In contrast to gas nitriding,    nitrogen and carbon are synchronously diffused in the valve port    plate 26 in the salt-bath nitriding, so that diffusion of nitrogen    is facilitated in comparison to the gas nitriding in which only    nitrogen is diffused. For gas nitriding, it is difficult to nitride    steel material which is not suitable for nitriding (that is, steel    other than nitriding steel). However, for the salt-bath nitriding,    it is easy to nitride (nitrocarburize) steel material (ferrous    material) other than nitriding steel. Additionally, it is possible    to nitride (nitrocarburize) steel material other than nitriding    steel by gas nitrocarburizing.-   (3) The valve port plate 26 is made of cold-rolled steel plate or    electromagnetic soft iron plate which is easy to machine in    comparison to nitriding steel.-   (4) The compressor 10 is of a piston type, having the cylinder block    11 and the piston 21 received in the cylinder bore 20 that is formed    in the cylinder block 11, in which refrigerant gas is introduced    into the cylinder bore 20 for compression therein and discharge    therefrom in conjunction with the reciprocation of the piston 21 in    the cylinder bore 20. In such piston type compressor, the discharge    chamber 25 is located relatively close to the suction chamber 24 in    comparison to other types of compressor, and the heat in the    discharge chamber 25 is easily transmitted through the valve port    plate 26 to the suction refrigerant gas in the suction. chamber 24.    However, by using a nitriding (nitrocarburizing) process which is    low in cost and easy to perform, an increase in temperature of    suction refrigerant gas due to the heat conduction is prevented.-   (5) The valve port plate 26 is located between the cylinder block 11    and the rear housing 14 which has formed therein the suction chamber    24 and the discharge chamber 25, and the nitride layers 26 a are    formed on the opposite front and rear surfaces of the valve port    plate 26. With the valve port plate 26 having on opposite surfaces    thereof the nitride layers, 26 a of substantially the same    thickness, an increase in temperature of the suction refrigerant gas    is prevented more effectively than with a valve port plate having a    nitride layer only on one surface thereof. To put in other words,    the suction valve plate 27 is disposed on the surface of the valve    port plate 26 on the side which is adjacent to the cylinder bore 20,    and the valve port plate 26 is exposed directly to the refrigerant    gas in the compression chamber 22 in the region of the valve port    plate 26 adjacent to the suction valve 31. Therefore, the valve port    plate 26 is exposed to the high-temperature refrigerant gas which is    compressed to a discharge pressure in the compression stroke, so    that the heat of the refrigerant gas is transmitted to the suction    port 29 through the contact portion and then to the suction    refrigerant gas. In the above-described preferred embodiment of the    present invention, however, the valve port plate 26 having the    nitride layers 26 a on both front and rear surfaces thereof can    prevent the heat transmission through the above path to the suction    refrigerant gas.-   (6) Carbon dioxide which is often used as refrigerant for vehicle    air conditioner has a higher refrigerating performance per unit    volume in comparison to fluorocarbon refrigerant, and the cylinder    bores of a compressor using such carbon dioxide refrigerant are made    smaller than those of a fluorocarbon refrigerant compressor,    accordingly. Therefore, when the refrigerant gas in the suction    chamber 24 expands by heating to reduce the amount of refrigerant    gas substantially introduced into the compression chambers 22, a    decrease in volumetric efficiency is large in percentage.    Accordingly, in the compressor 10 using carbon dioxide refrigerant,    the improvement in volumetric efficiency by preventing the expansion    of refrigerant gas due to heating of the suction refrigerant gas is    larger than that of a fluorocarbon refrigerant compressor. Thus, the    present invention is particularly suitable for the compressor 10    which is designed for compressing carbon dioxide refrigerant.-   (7) When the valve port plate 26 is nitrided (nitrocarburized), the    base material 37 has formed on the surface thereof the thin and hard    nitride layer 26 a, while having formed therein the diffusion layer    37 a of nitrogen contiguously to the nitride layer 26 a. Thus, the    valve port plate 26 will have higher abrasion resistance and better    bedding-in pattern.

The present invention is not limited to the embodiments described abovebut may be modified into the following alternative embodiments.

In an alternative embodiment, the valve port plate 26 is nitrided ornitrocarburized to have a thermal conductivity of 60 W/mK or less. Forexample, as shown in FIG. 6, the nitride layer 26 a is formed only onthe rear surface of the valve port plate 26 adjacent to the rear housing14. FIG. 7 shows another alternative embodiment wherein the nitridelayer 26 a is formed only on the front surface of the valve port plate26 adjacent to the cylinder block 11. If the nitride layer 26 is formedonly on one surface of the valve port plate 26, the nitride layer 26 ais preferably formed on the rear surface of the valve port plate 26adjacent to the rear housing 14 because the rear surface of the valveport plate 26 has a larger area exposed to discharged gas.

The present invention is not limited to the valve port plate 26 which isnitrided or nitrocarburized to have a thermal conductivity of about 60W/mK or less, for which, for example, the valve port plate 26 needs tobe salt-bath nitrided to have the nitride layer 26 a having a thicknessof about 20 μm or more. Alternatively, for example, the valve port plate26 needs to be gas nitrocarburized to have the nitride layer 26 a havinga thickness of about 50 μm or more according to the results shown inFIG. 5. In an alternative embodiment, the valve port plate 26 issalt-bath nitrided or gas nitrocarburized to have the nitride layer 26 ahaving a thickness of about 10 μm or more, which shows a decrease inthermal conductivity of the valve port plate 26 according to the resultsshown in FIG. 5.

In an alternative embodiment, the suction chamber 24 is formed inradially outer region of the rear housing 13, while the dischargechamber 25 is formed in radially inner region, as shown in FIG. 8.

The nitriding or nitrocarburizing is not limited to the salt-bathnitriding or gas nitrocarburizing. In an alternative embodiment, othernitriding processes are usable. Other nitriding processes include gasnitriding and ion nitriding (plasma nitriding). The relation between athickness of the nitride layer 26 a and a thermal conductivity of thevalve port plate 26 depends on which nitriding or nitrocarburizing isperformed for forming the nitride layer 26 a. Therefore, a thickness ofnitride layer 26 a required for gaining a target thermal conductivity ofthe valve port plate 26 is determined adequately depending on whichnitriding or nitrocarburizing is performed.

The material for the valve port plate 26 is not limited to cold-rolledsteel plate and electromagnetic soft iron plate, but any ferrousmaterial is usable. For example, hot-rolled mild steel plate ornitriding steel is usable. Additionally, the nitriding steel may benitrided (nitrocarburized) easier than other ferrous material.

The present invention is not limited to the above-described swash platetype variable displacement compressor, but it is also applicable to aswash plate type compressor with a double-headed piston or a fixeddisplacement. Additionally, the compressor of the present invention maybe of a wobble type in which the swash plate wobbles with the rotationof the drive shaft without making integral rotation with the driveshaft.

In an alternative embodiment, the housing of the compressor 10 is notlimited to the structure that the front housing 12 and the rear housing14 hold the cylinder block 11 therebetween. For example, the housing ofthe compressor includes a front housing and a rear housing, and eitherone of the front and rear housings has formed therein a crank chamber,while the other receives therein a cylinder that has formed therein acylinder bore.

Alternatively, the present invention is applicable to a compressorhaving a piston which is operated by means other than the swash plate.Additionally, the present invention is not limited to a piston typecompressor but is usable for a scroll type compressor.

The present invention is not limited to a compressor that uses carbondioxide as refrigerant for vehicle air conditioner but is usable for acompressor that uses fluorocarbon refrigerant.

The present invention is not limited to a compressor whose drive shaftis rotated by the power of the engine, but the drive shaft of thecompressor may be driven by a motor.

In an alternative embodiment, the compressor is not limited to be usedfor a vehicle air conditioner but may be a motor compressor that is usedfor a domestic air conditioner.

The present invention is not limited to a compressor used for airconditioning, but it is applicable to a compressor for other refrigerantcircuits, such as a compressor used for a refrigerant circuit of arefrigerator or a freezer.

Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein but may be modified within the scope of theappended claims.

1. A compressor comprising: a valve port plate made of a steel, whereinthe valve port plate is nitrided or nitrocarburized.
 2. The compressoraccording to claim 1, wherein the valve port plate has a thermalconductivity of about 60 W/mK or less.
 3. The compressor according toclaim 1, wherein the nitriding or nitrocarburizing is selected from agroup comprising salt-bath nitriding, gas nitrocarburizing, gasnitriding and ion nitriding.
 4. The compressor according to claim 1,wherein the valve port plate is made of nitriding steel.
 5. Thecompressor according to claim 1, wherein the valve port plate is made ofelectromagnetic soft iron.
 6. The compressor according to claim 1,wherein the valve port plate is made of cold-rolled steel plate.
 7. Thecompressor according to claim 1, wherein the valve port plate is made ofhot-rolled mild steel plate.
 8. The compressor according to claim 1,wherein the compressor is of a piston type, further comprising: acylinder block having formed therethrough a cylinder bore; and a pistonreceived in the cylinder bore for reciprocation therein, whereby gas isintroduced, compressed and discharged.
 9. The compressor according toclaim 8, wherein the gas is refrigerant gas.
 10. The compressoraccording to claim 8, further comprising: a housing having formedtherein a suction chamber and a discharge chamber, wherein the valveport plate is located between the housing and the cylinder block, andwherein a nitride layer is formed on at least one surface of the valveport plate.
 11. The compressor according to claim 10, wherein thenitride layer is formed on the surface of the valve port plate adjacentto the housing.
 12. The compressor according to claim 10, wherein thenitride layer is formed on the surface of the valve port plate adjacentto the cylinder block.
 13. The compressor according to claim 10, whereinthe nitride layer is formed on opposite surfaces of the valve portplate.
 14. The compressor according to claim 10, wherein the valve portplate has formed therein a diffusion layer of nitrogen contiguously tothe nitride layer.
 15. The compressor according to claim 1, wherein thevalve port plate is salt-bath nitrided or gas nitrocarburized to have anitride layer having a thickness of about 10 μm or more.
 16. Thecompressor according to claim 15, wherein the nitride layer has athickness of about 20 μm or more.
 17. The compressor according to claim15, wherein the valve port plate is gas nitrocarburized to have anitride layer having a thickness of about 50 μm or more.
 18. Thecompressor according to claim 1, wherein carbon dioxide is used asrefrigerant for the compressor.